WANs (Wide Area Networks) based on, for example, IP-VPN technology are generally used as the communication network between global locations. When a terminal at a specific location makes communication with another terminal at a different oversea location, the respective locations are connected by a WAN for communication. In the WAN, the available bandwidth is usually limited by the contract. Overseas communication especially has a significant delay.
TCP is generally used for communication between terminals. TCP controls the bandwidth of communication according to the magnitude of RTT (Round Trip Time). The high throughput may be difficult in communication via the WAN. The technique disclosed in PTL1 has been proposed as the technology enabling the high throughput.